1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device using a NAND-type EEPROM in an electrically erasable programmable nonvolatile semiconductor memory (EEPROM).
2. Description of the Related Art
In a computer system, a programmable memory device (or memory) generally has a physically limited capacity and is used such that new data is overwritten on unnecessary data. Programmable memory devices (memories) can be classified into two types of programmable memory devices according to overwrite methods. In one type of memory device, as in a random access memory (RAM), a hard disk, a floppy disk, or a magnetic tape, new data can be directly overwritten on old data. In the other type of memory device, as in a certain type of optical memory device or an EEPROM, after old data on which new data is to be overwritten is erased, the new data can be written.
A NAND-type EEPROM has two types of erasing methods. According to the first method, as in, e.g., a flash EEPROM available from INTEL CORP., data on an entire chip is erased at once. According to the second method, only data on part of a chip is selectively erased.
In the NAND-type EEPROM, a plurality of structurally continued memory cells for continuously reading data or writing data is called a page as a unit. For example, in a 4-Mbit EEPROM, one page is constituted by memory cells having 4096 bits. The plurality of structurally continued pages are called a block as a unit. For example, in a 4-Mbit EEPROM, one block is constituted by memory cells of 8 pages (4 kbytes). In a NAND-type EEPROM, a unit used when only data on part of one chip is selectively erased coincides with the block.
Since the NAND-type EEPROM can partially erase data as described above, the NAND-type EEPROM is a nonvolatile semiconductor memory capable of relatively easily performing an operation in which data of one sector can be rewritten. Therefore, the characteristic features of a semiconductor memory such as reliability related to a mechanical strength, low power consumption, and a short read access time are effectively utilized, and the NAND-type EEPROM has been used in applications where a conventional magnetic disk device is taken over.
However, although an access time for reading out data is short in the EEPROM, an access time for writing data is long. For example, in a 4-Mbit NAND-type EEPROM, although a time required for reading out data of one block is about 490 .mu.sec, and times required for erasing data of one block and writing new data of one block are about 10 msec for erasure and about 4 msec for write access.
In addition, according to the current technique, a maximum rewrite count is limited. When a rewriting operation is performed 10.sup.4 or 10.sup.5 times, the EEPROM reaches the end of its service life. Therefore, when an overwriting operation of data is concentrated on the same block, the service life of an entire chip is shortened.
As in file management performed by an OS, a file may be generally recorded in any empty area of a data area, and the empty area is managed by a directory and a management table such as a File Allocation Table (FAT). This file management is applied to an EEPROM having a limited maximum rewrite count, and positions in which files are to be written can be easily distributed to prevent a rewriting operation from being concentrated on the same position. However, when a file is written or rewritten, the content of a file management table must be rewritten, and the frequency of rewriting of these table data is considerably higher than that of writing of data files. Furthermore, the content of the management table must always be recorded on a fixed position. Therefore, when an EEPROM is applied to a file memory device, a rewriting operation is concentrated on the area of the management table area, and the service life of the EEPROM may be shortened.
The related arts of the present invention are disclosed in Published Unexamined Japanese Patent Application No. 2-292798 and Published Unexamined Japanese Patent Application No. 3-167644.